Human platelet-derived growth factor ("PDGF") is believed to be the major mitogenic growth factor in serum for connective tissue cells. The mitogenic activity of PDGF has been documented in numerous studies, wherein PDGF has been shown to positively affect mitogenesis in arterial smooth muscle cells, fibroblast cells lines, and glial cells. Deuel et al., J. Biol. Chem., 256(17), 8896-8899 (1981). See also, e.g., Heldin et al., J. Cell Physiol., 105, 235 (1980) (brain glial cells); Raines and Ross, J. Biol. Chem., 257, 5154 (1982) (monkey arterial smooth muscle cells). PDGF is also believed to be a chemoattractant for fibroblasts, smooth muscle cells, monocytes, and granulocytes. Because of its apparent abilities to both induce mitogenesis at the site of connective tissue wounds, and to attract fibroblasts to the site of such wounds, PDGF is thought to have particular potential for therapeutic use in the repair of injured, or traumatized, connective tissues.
Other members of the PDGF family include vascular endothelial cell growth factor ("VEGF", sometimes also referred to as "vascular permeability factor, or "VPF") and placental growth factor ("PLGF"). Tischer et al., Biochem. Biophys. Res. Comm., 165(3), 1198-1206 (1989) and Maglione et al., Proc. Natl. Acad. Sci. USA, 88, 9267-9271 (1991), respectively. Both VEGF and PLGF form disulfide bonded dimers from the eight highly conserved cysteine residues that appear in the PDGF homologous region of each monomeric unit of these PDGF family members. Tischer et al. and Maglione et al., ibid. The receptors for VEGF and PLGF are also in the same receptor subfamily as the PDGF receptors. Consequently, these "newer" members of the PDGF family are thought to be potentially useful as therapeutic products in wound repair, although they have not been studied as extensively as PDGF.
Naturally occurring PDGF is a disulfide-bonded dimer having two polypeptide chains, namely the "A" and "B" chains, with the A chain being approximately 60% homologous to the B chain. Naturally occurring PDGF is found in three dimeric forms, namely PDGF-AB heterodimer, PDGF-BB homodimer, or PDGF-AA homodimer. Hannink et al., Mol. Cell. Biol., 6, 1304-1314 (1986). Although PDGF-AB has been identified as the predominate naturally occurring form, it is the PDGF-BB homodimer that has been most widely used in wound healing studies. Each monomeric subunit of the biologically active dimer, irrespective of whether it is an A chain monomer or a B chain monomer, contains eight cysteine residues. Some of these cysteine residues form interchain disulfide bonds which hold the dimer together.
The PDGF-B found in human platelets has been identified as a 109 amino acid cleavage product (PDGF-B.sub.109) of a 241 amino acid precursor polypeptide Johnsson et al., EMBO Journal, 3(5), 921-928 (1984). This 109 amino acid homologous sequence coincides with the 109 amino acid cleavage product of the c-sis encoded PDGF-B precursor protein and is believed by many to be the mature form of PDGF in humans. Homology with the c-sis encoded precursor protein begins at amino acid 82 of the 241 amino acid precursor protein and continues for 109 amino acids. Another form of PDGF-B (PDGF-B.sub.119), corresponding to the first 119 amino acids of the c-sis encoded PDGF-B precursor protein, has also been identified as a major cleavage product of the c-sis encoded precursor protein when the entire c-sis gene is encoded into a transfected mammalian host. U.S Pat. No. 5,149,792.
The application of PDGF to dermal wounds, including incisional wounds and dermal ulcers, in human and/or animals has been shown to accelerate the rate at which these types of wounds heal. Pierce et al, 167, J. Exp. Med., 974-987 (1988) (incisional wounds in rats); Robson et al., The Lancet, 339, 23-25 (1992) (human dermal ulcers). PDGF has is believed to function in the acceleration of wound healing by stimulating the deposition of a provisional matrix in the wound bed. Pierce et al., Am. J. Pathology, 140(6), 1375-1388 (1992). PDGF is also believed to indirectly stimulate supportive angiogenesis in connection with the deposition of this provisional matrix. Pierce et al., ibid. However, this degree of supportive angiogenesis may be insufficient for PDGF alone to significantly accelerate the healing of ischemic wounds. Pierce et al., ibid. More importantly, PDGF has not demonstrated an ability to create collateral circulation in ischemic tissue at risk of necrosis.
Currently, the best method for providing collateral circulation to tissue at risk of ischemic necrosis is surgical anastomosis, or bypass surgery. Although advances in endoscopic technology have made it possible to perform some cardiac surgical procedures through a thoracoscope (Mack et al., Ann. Thorac. Surg., 56, 739-740 (1993); Hazelrigg et al., Ann. Thorac. Surg., 56, 792-795 (1993); Frumin et al., PACE, 16, 257-260 (1993)), it is not possible to perform coronary artery bypass grafting through a thoracoscope without cardiopulmonary bypass unless and until coronary artery anastomoses can be performed reliably and safely on a beating heart.
Thus, there are no established noninvasive procedures for creating collateral circulation in tissue at risk of ischemia. Nevertheless, fibroblast growth factor (FGF), considered to be the prototypical angiogenic agent, has been suggested in the treatment of ischemic heart disease and to alleviate conditions caused by myocardial infarction. U.S. Pat. Nos. 4,278,347 and 4,296,100, respectively. Yanagisawa et al. Science, 257, 1401-1403 (1992) have also injected basic FGF into the proximal coronary circulation and demonstrated a reduction in the size of myocardial infarction upon coronary occlusion. This approach, however, relies upon good blood flow in the very same coronary circulation that has already been compromised by the atherosclerotic disease process. Unger et al., Am. J. Physiol., 264, H1567--H1574 (1993), reported that implanting the internal mammary vessels into the left ventricular myocardium, with infusion of acidic FGF into the distal ends of the implanted vessels, resulted in no beneficial effect beyond that which was achieved with heparin infusion alone.
It is an object of the present invention to provide a noninvasive method for improving collateral circulation in tissue at risk of ischemia-or ischemic necrosis.
It is a further object of the present invention to provide a noninvasive method to effect the anastomosis of blood vessels.
It is a still further object of the present invention to provide a nonsurgical method for conducting bypass grafting.